Datasheet

LM49450
SNAS440D –FEBRUARY 2008–REVISED MAY 2013 www.ti.com
GENERAL AMPLIFIER FUNCTION
Class D Amplifier
The LM49450 features a high-efficiency stereo Class D audio power amplifier that utilizes TI’s filterless
modulation scheme which reduces external component count, conserves board space and reduces system cost.
The Class D outputs transition between V
DD
and GND with a 300kHz switching frequency. With no signal
applied, the outputs switch with a 50% duty cycle, in phase, causing the two outputs to cancel. This cancellation
results in no net voltage across the speaker, thus there is no current to the load in the idle state.
With the input signal applied, the duty cycle (pulse width) of the LM49450 outputs changes. For increasing output
voltage, the duty cycle of V_LS+ increases while the duty cycle of V_LS- decreases. For decreasing output
voltages, the converse occurs. The difference between the two pulse widths yield the differential output voltage.
Fixed Frequency Mode
The LM49450 features two modulation schemes, a fixed frequency mode and a spread spectrum mode. Select
the fixed frequency mode by setting the SS bit (B3) in the Mode Control Register (0x00h) to 0. In fixed frequency
mode, the speaker amplifier outputs switch at a constant 300kHz. The output spectrum in fixed frequency mode
consists of the fundamental and its associated harmonics (see Typical Performance Characteristics).
Spread Spectrum
The logic selectable spread spectrum mode eliminates the need for output filters, ferrite beads or chokes. In
spread spectrum mode, the switching frequency varies randomly by 30% about a 300kHz center frequency,
reducing the wide-band spectral content, improving EMI emissions radiated by the speaker and associated
cables and traces. Where a fixed frequency class D exhibits large amounts of spectral energy at multiples of the
switching frequency, the spread spectrum architecture of the LM49450 spreads that energy over a larger
bandwidth (see Typical Performance Characteristics). The cycle-to-cycle variation of the switching period does
not affect the audio reproduction, efficiency, or PSRR. Set the SS bit (B3) in the Mode Control Register (0x00h)
to 1 to select spread spectrum mode.
Headphone Amplifier
The LM49450 headphone amplifiers feature TI’s ground referenced architecture that eliminates the large DC-
blocking capacitors required at the outputs of traditional headphone amplifiers. A low-noise inverting charge
pump creates a negative supply (HPV
SS
) from the positive supply voltage (CPV
DD
). The headphone amplifiers
operate from these bipolar supplies, with the amplifier outputs biased about GND, instead of a nominal DC
voltage (typically V
DD
/2), like traditional amplifiers. Because there is no DC component to the headphone output
signals, the large DC-blocking capacitors (typically 220µF) are not necessary, conserving board space and
system cost, while improving frequency response.
Power Supplies
The LM49450 uses different power supplies for each portion of the device, allowing for the optimum combination
of headroom, power dissipation and noise immunity. The analog input, and gain (volume control) stages for both
speaker and headphones are powered from V
DD
. The speaker output stage is powered from LSV
DD
. The
headphone amplifiers and charge pump are powered from HPV
DD
. The separate power supplies allow the class
D amplifiers to operate from a higher voltage, maximizing headroom, while the headphones operate from a lower
voltage, improving power dissipation, as well as minimizing switching noise coupling between the speaker and
headphone amplifiers. The digital portion of the device is powered from DV
DD
, including the 3D processing core
and DAC. IOV
DD
powers the I
2
S and I
2
C, allowing the LM49450 to interface with lower voltage digital controllers.
TI's 3D Enhancement
The LM49450 digital audio path features TI’s 3D enhancement that widens or narrows the perceived soundstage
of a stereo audio signal. The 3D enhancement either increases or decreases the apparent stereo channel
separation, improving audio reproduction whenever the placement of both left and right speakers is not ideal.
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